Custom element (with set-transform)'s Anchor position error

[johannes-wolf]

Discussed in #699

^{Originally posted by KiyanYang September 24, 2024}
I created a custom element, invoke set-transform in the element，and set Anchor. Externally, call this element's Anchor position error. Is there something wrong with the code?

In the picture below, red line should link point A to point D1, but D1 position wrong.

#import "@preview/cetz:0.2.2"

#set page(width: auto, height: auto)

#let prism(name: none, show-content: false, ..style) = {
  import cetz.draw: *
  group(name: name, ctx => {
    let default_style = (
      edge-length: (2cm, 3cm, 2cm),
      thickness: 1pt,
      transform: (
        (-calc.sqrt(2)/4, 1, 0, 0),
        ( calc.sqrt(2)/4, 0,-1, 0),
        (              0, 0, 1, 0),
        (              0, 0, 0, 1)))
    let style = cetz.styles.resolve(ctx.style, merge: style.named(), base: default_style, root: "prism")

    set-transform(style.transform)

    let a = style.edge-length.at(0);
    let b = style.edge-length.at(1);
    let c = style.edge-length.at(2);
    anchor("A", (a,0,0))
    anchor("B", (a,b,0))
    anchor("C", (0,b,0))
    anchor("D", (0,0,0))
    anchor("A1", (a,0,c))
    anchor("B1", (a,b,c))
    anchor("C1", (0,b,c))
    anchor("D1", (0,0,c))

    set-style(stroke: (thickness: style.thickness, cap: "round"))
    line("A", "B")
    line("B", "C")
    line("C", "D", stroke: (dash: "dashed"))
    line("D", "A", stroke: (dash: "dashed"))
    line("A", "A1")
    line("B", "B1")
    line("C", "C1")
    line("D", "D1", stroke: (dash: "dashed"))
    line("A1", "B1")
    line("B1", "C1")
    line("C1", "D1")
    line("D1", "A1")

    if show-content {
      content("A",  $A$,   padding: (left: -4mm, bottom:  0mm))
      content("B",  $B$,   padding: (left:  4mm, bottom: -1mm))
      content("C",  $C$,   padding: (left:  4mm, bottom:  0mm))
      content("D",  $D$,   padding: (left: -4mm, bottom:  1mm))
      content("A1", $A_1$, padding: (left: -5mm, bottom:  0mm))
      content("B1", $B_1$, padding: (left: -4mm, bottom:  6mm))
      content("C1", $C_1$, padding: (left:  0mm, bottom:  6mm))
      content("D1", $D_1$, padding: (left:  0mm, bottom:  6mm))
    }
  })
}

#cetz.canvas({
  prism(name: "test", show-content: true)
  cetz.draw.line("test.A", "test.D1", stroke: red) // Wrong location
})
```</div>

[johannes-wolf]

@KiyanYang The problem seems to be the z component.
It works if you remove it from your transformation matrix:

transform: (                                         
        (-calc.sqrt(2)/4, 1, 0, 0),                        
        ( calc.sqrt(2)/4, 0,-1, 0),                        
        (              0, 0, 0, 0), // Set z to 0                        
        (              0, 0, 0, 1))

Because the default transformation matrix sets z * (1/2, 1/2, 0), you have to either change the root transformation or remove the z component from the resulting vectors of your transformation.

It is not good that anchors behave like this, though. I will have a look at it.